A steel strip, especially a coated steel strip obtained by coating a specific molten metal, e.g., molten zinc onto a cold-rolled steel strip, is highly corrosion-resistant and has aesthetic appearance.
Especially, lately, this coated steel strip has been utilized in electronic products or automobiles, thus calling for development of a manufacturing method for a higher-quality coated steel strip.
The steel strip is chiefly coated by virtue of continuous hot dip galvanizing.
For example, as shown in FIG. 1, a coil steel strip (cold-rolled steel strip) S uncoiled from a pay off reel is thermally treated in a furnace through a welder and an entry looper. Then the coil steel strip S passes through a hot dipping bath 110 filled with molten zinc 112 through a snout 114 to be coated.
Next, the steel sheet passes through a gas wiping apparatus 120 (or an air knife) disposed over a molten level of the hot dipping bath. In this case, the molten metal solution (zinc) of the steel strip S is adequately worn from surfaces thereof by a high-pressure air or a non-active gas such as nitrogen, which will be hereinafter referred to as ‘gas’, blown onto the steel strip. This allows the steel strip to be adjusted in its coating thickness as shown in A of FIG. 1.
Afterwards, a gauge 130 measures whether the steel strip is coated to an appropriate coating thickness. The measured value is fed back to adjust a gas ejection pressure of the gas wiping apparatus 120 and an interval between the steel strip S and the gas wiping apparatus 120, thereby continuously controlling a coating amount of the steel strip.
Here, reference signs 116 and 118 in FIG. 1, which are not described, indicate a sink roll for guiding the steel strip into the hot dipping bath and a stabilizing roll for suppressing vibration of the steel strip.
As described above, the gas wiping apparatus (air knife) 120 is the important equipment for determining a coating thickness of the steel strip to meet consumer's demand.
FIG. 2 illustrates splashing of a molten metal solution (molten zinc) which occurs in a gas wiping apparatus 120.
As shown in FIG. 2, a high-speed high-pressure gas is ejected through outlets 122 e.g., slits formed between upper and lower lips of the gas wiping apparatus 120, to collide against surfaces of the steel strip. Here, molten metal particles, e.g., molten zinc particles, (which will be hereinafter referred to as zinc chips) deposited on the steel strip are splashed due to gas moving upward and downward at a high speed.
For example, in FIG. 2, the gas ejected from the gas outlets at a high pressure and a high speed causes a negative pressure area to be formed around the outlets owing to the gas traveling fast so that the zinc chips d are splashed from the surfaces of the coated steel strip.
Meanwhile, currently, to boost production of the coated steel strip, the steel strip is made to move at a higher speed. Also, to lower costs of the steel strip, a molten metal layer is required to be coated on the steel strip to a thickness as small as possible within a desired extent.
In the case of thin coating, greater portions of the molten metal should be worn from the surfaces of the coated steel strip passing through the hot dipping bath as indicated with reference numeral 110 of FIG. 1. Therefore, this requires gas to move at a higher speed, thereby increasing a collision pressure.
However, as described above, the higher speed of the gas ejected proportionally aggravates splashing of the zinc chips so that the gas is limitedly increased in its ejection speed.
The zinc chips d splashed as described above are deposited on or around the outlets of the gas wiping apparatus, causing the gas to be ejected with non-uniform distribution along a width direction of the steel strip. This results in defective coating of the steel strip.
Therefore, to restrain the molten metal from splashing during thin coating, currently the steel strip moves at a lower speed to be coated, thus limiting a high-speed travel of the steel strip and eventually degrading production of the steel strip.
Lately, a major concern with the gas wiping apparatus 120 is to eject the gas at a high speed and a high pressure while moving the steel strip faster and maximally suppressing the zinc chips from splashing which adversely affects product quality.
Conventionally, several technologies with regards to the gas wiping apparatus have been proposed to suppress the zinc chips from splashing. However, these technologies entail a complicated separate structure other than the gas wiping apparatus. Also, with the technologies, the zinc chips are not effectively prevented from splashing in an actual coating process.